1. Field of the Invention
The present invention relates to an image display apparatus such as a front projector or a rear projector.
2. Description of the Related Art
In general, an image display apparatus employs a liquid crystal display panel or a micro mirror array device as a light modulation element and controls transmission and shielding of light or polarization, such that a selected optical pattern is projected by a projection optical system, to thereby display an image on a projection surface.
Examples of an illumination device conventionally used for the image display apparatus include a high-intensity discharge lamp such as an extra high pressure mercury lamp, a metal halide lamp, or a xenon lamp, and the image display apparatus typically includes a paraboloidal surface reflector which collimates illumination light beams from the above-mentioned light sources. However, when the high-intensity discharge lamp is used, there arise problems that it is necessary to provide a cooling system for dealing with heat generated by the high-intensity discharge lamp, which increases a size of the illumination device, and the high-intensity discharge lamp has a short life of several thousand hours.
In view of the above, a light source using a semiconductor is drawing attention in recent years as a new light source. Particularly, there has been made such a significant improvement on a light emitting diode (LED) that a product having intensity and efficiency high enough to be used as lighting is being developed. For example, Japanese Patent Application Laid-open No. 2001-185760 discloses a reflective LED package in which a concave reflecting surface 2 is provided behind an LED 1. Also, Japanese Patent Application Laid-open No. 2003-234513 discloses a cannonball-type LED package obtained by molding an LED 3 with a synthetic resin material 4 in a lens shape. The LED has advantages over the above-mentioned high-intensity discharge lamp in terms of size reduction, weight reduction, low-power consumption, extended life, and quick response when turned on.
However, it is difficult at present to attain sufficient luminance on a screen in an image display apparatus having an LED as a light source. This is because the LED is still inferior to the extra high pressure mercury lamp in terms of efficiency, and only a small amount of light can be obtained from one LED even with a current maximum to the rating. For this reason, there has been a method of arranging a plurality of LEDs on a plane in an array in order to increase light amount. For example, Japanese Patent Application Laid-open No. 2001-281760 discloses a display device in which a plurality of solid-state light emitting elements are arranged in a matrix, and an integrator illumination is adopted to superimpose light beams from the plurality of solid-state light emitting elements on a light modulation element, to thereby increase luminance. Also, for example, Japanese Patent Application Laid-open No. 2004-286858 discloses a lighting device in which a plurality of solid-state light emitting elements are two-dimensionally arranged and light emitting surfaces of the solid-state light emitting elements are arranged such that the light emitting surfaces are in conjugate relation with a light modulation element, so an enlarged image in the light emitting surfaces is superimposed on the light modulation element, to thereby attain uniform and high-intensity lighting.
Also, in the conventional image display apparatus, as disclosed in U.S. Pat. No. 6,989,939, an aperture stop is disposed in a secondary light source plane in proximity to an integrator lens such as a fly eye lens in an illumination optical system so as to restrict light beams reaching a projection surface which is a conjugate surface with the aperture stop. With this structure, a situation is prevented in which unnecessary light from a light modulation element is subjected to total reflection in a prism system such as a polarization beam splitter to form a ghost on the projection surface.
Also, U.S. Pat. No. 7,016,124 discloses a system as illustrated in FIG. 23 in which an aperture stop 5 has a shape corresponding to a shape of each cell 6 constituting an integrator lens, and further, the aperture stop 5 is made variable in size. With this structure, an illuminance distribution on a projection surface is made uniform and brightness and contrast of an image can be adjusted.
As regards a projection optical system used for an image display apparatus such as a front projector or a rear projector, there has been conventionally well known a projection optical system as disclosed in U.S. Pat. No. 6,989,939 which uses a wide-angle lens of a coaxial optical system in which an optical axis connecting a center of an object plane and a center of an image plane is not bent.
However, in recent years, there has been proposed a projection optical system as disclosed in U.S. Pat. No. 7,016,124 which is called an off-axial optical system, in which an optical axis is bent and the constituent planes are non-coaxial. In the off-axial optical system, a keystone distortion occurring when an image is obliquely projected can be corrected and the projection range can be easily increased as compared with a coaxial optical system. U.S. Pat. No. 7,016,124 describes in detail characteristics and advantages of the off-axial optical system.
The off-axial optical system using a reflecting surface has many advantageous characteristics in comparison with the conventional coaxial optical system, but the off-axial optical system is often extremely sensitive to product deviations such as surface accuracy or decentering, which leads to a problem in mass production. For example, when an asymmetric deviation is generated in a plane shape, an image plane goes out of focus in two directions orthogonal to each other across the entire image plane, that is, an astigmatic difference is generated, which is hard to eliminate by mechanically adjusting the system.
To deal with the above-mentioned problem, U.S. Pat. No. 7,016,124 discloses a structure in which a pupil of the off-axial optical system is made asymmetric in an off-axial cross section and a cross section orthogonal to the off-axial cross section. In other words, to use the off-axial optical system using a reflecting surface, as a projection optical system of an image display apparatus, an illumination optical system is indispensable for attaining the pupil shape without reducing illumination efficiency in the entire image display apparatus.
However, in the image display apparatus which uses a conventional combination of a high-intensity discharge lamp and a reflector as a light source, a projection optical system having a rotationally symmetric stop shape is generally adopted because an aperture of the reflector is substantially circular. Alternatively, the image display apparatus may adopt a method in which a cylindrical lens is provided in the illumination optical system to compress illumination light beams in a desired direction, to thereby form the stop of the projection optical system into an asymmetric shape.
An optical system conserves a product of an area of the light source and a divergence solid angle of the light beams (conserves etendue), so there is a limit in controlling a stop shape of the projection optical system. In order to lift the limit, the light source itself may be formed into a rotationally asymmetric shape. In order to form the light source into a rotationally asymmetric shape, it is necessary to arrange small solid-state light emitting elements in an array so as to form a shape corresponding to a desired stop shape of the projection optical system.
According to Japanese Patent Application Laid-open Nos. 2001-281760 and 2004-286858, solid-state light emitting elements are arranged in an array mainly for the purpose of improving luminance and reducing unevenness in luminance on the light modulation element. There is no description on the relation between the light source and the pupil or the stop of the projection optical system and it is not intended to attain an effect of controlling the stop shape of the projection optical system. Therefore, the inventions disclosed in Japanese Patent Application Laid-open Nos. 2001-281760 and 2004-286858 are different from the present invention in effect.
Further, U.S. Pat. Nos. 6,989,939 and 7,016,124 disclose a structure in which an aperture stop is disposed in proximity to the integrator lens, to thereby restrict light beams. However, this structure reduces illumination efficiency and increases unevenness in illuminance on the projection surface because cells constituting the integrator lens are partially light-shielded only on the periphery.